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Bubble Production in Divers Who Have Had DCS

Venous gas embolism (VGE), or bubbles, in divers postdive indicates that their decompression was too fast, their bodies became supersaturated and free gas emerged from solution in tissues. The occurrence of free gas is considered a necessary condition for decompression sickness (DCS), which can happen even without VGE. However, the presence of VGE increases the number and types of possible harms to the body and thus the probability of DCS.

A number of studies indicate variability in proneness to DCS among divers; however, the question of whether divers who have suffered DCS produce bubbles more readily in general has not been answered yet. To answer this question, researchers would need to identify “bubblers” and “nonbubblers” and observe the outcomes of their dives over some period of time, which would require a lot of resources and time.

Researchers from Freemantle Hospital in Australia conducted an observational cohort study that would provide some answers more expeditiously.1 They identified a group of divers who have suffered DCS in the past and a control group of divers who have never had DCS; they exposed both groups to the same dive in a hyperbaric chamber to 405 kPa (30 msw, 99 fsw) for a 15-minute bottom time and decompressed after that on Defence and Civil Institute of Environmental Medicine (DCIEM) tables, which are renowned for their safety. After decompression researchers used echocardiography to monitor VGE in the divers. They qualitatively evaluated the quantity of bubbles using a scale of 0 (no bubbles) to 5 (flood of bubbles; a single bubble cannot be discriminated).

It is important to note that divers with a history of severe DCS or arterial gas embolism were excluded from the DCS group, and divers with diagnosed patent foramen ovale (PFO) were excluded from both groups.

The authors claim the findings indicate that divers with a history of DCS had a consistently higher degree of VGE. Reading the results, however, is confusing. The study originally enrolled 24 divers: 11 with a history of DCS and 13 without. It appears that during the study some of the enrolled subjects did not meet the criteria and were cut for some purposes but not for others. For example, the total for one table adds to 18 (7 DCS and 11 non-DCS) divers, and the other adds to 19 (6 DCS and 13 non-DCS) divers. The sample size was small enough to show raw results, but instead the statistics were not used in a very transparent way.

While we commend the authors for opening this question, the sample size was obviously too small to answer it. It is unfortunate that the study excluded divers with a history of severe DCS and skin DCS; the justification for that exclusion is not convincing. Similarly, there is no need to exclude subjects with a diagnosed PFO: The difference in proneness to bubbles may correlate to proneness of DCS in divers with a PFO. Finally, not using dynamic maneuvers to mobilize bubbles from the extremities is not completely justifiable because the study was comparing divers who got DCS after real dives that typically include such activities and not after simulated dives with a prescribed resting period upon surfacing.

The authors plan to increase the sample size in the future. We also suggest that they present the number of dives the divers have made in their lifetime. Each diver has been a member of the non-DCS group, at least for a part of their dive careers, unless they got hit on their first dive. Increasing the sample size and providing some measure of exposure would strengthen the study.